Endocrine functions of the pancreas gland are performed by a multi-cellular structure known as the islets of Langerhans that are present throughout the pancreas gland. These structures are also known as pancreatic islets, beta islets or pancreatic beta islets. Islets of the adult pancreas are comprised of alpha, beta, delta, pancreatic polypeptide (PP) cells, which are endocrine cells that synthesize and release glucagon, insulin, somatostatin and pancreatic polypeptide respectively. Also, the beta islets contain fibroblast and endothelial cells, the latter comprising blood vessels that permeate the islets. Both insulin and glucagon mediate central roles in carbohydrate metabolism: insulin through promotion of cellular uptake and utilization of glucose and glucagon by its glycogenolytic effect that generates glucose from glycogen.
Type I diabetes mellitus occurs by selective autoimmune destruction of the insulin-producing beta cells of the beta islets. In the late 1990's, Canadian scientists discovered that transplantation of purified beta islets into diabetic patients resulted in insulin independence when islets from at least two donors were transplanted using specific anti-rejection drugs (Shapiro, et al, 2000). The so-called Edmonton protocol has been subject to additional clinical trials now including about 300 transplant recipients. More than half of the transplant recipients were insulin independent one year following transplantation and there was also a significant reduction in diabetes-related complications in the transplant recipients. These findings provide encouragement for finding an effective treatment of type I diabetes mellitus, a debilitating disease affecting about 15 million people worldwide with an increasing rate of incidence. While insulin therapy can regulate blood glucose to normal levels, it is not a cure and many of the life threatening consequences of diabetes eventually strike insulin-treated type I diabetics with enormous medical costs, reducing quality of life and resulting in premature death. Further development of transplantation therapy depends on generation of much larger quantities of human beta cells than are available through organ donation and this is a major goal of diabetes research.
One approach to developing large quantities of beta islets for transplantation is the development of methods to induce stem cells to develop into fully differentiated beta islets suitable for transplantation. Studies of the proliferative capacity of beta islets through in-vitro cell culture indicate that islets do not proliferate and rapidly lose functional properties including insulin content and its secretion in response to elevated glucose concentrations (McEvoy, et al, 1986). There is thus a need for an in-vitro model system of self-renewing stem cells that can be induced to form fully differentiated beta islets, including the constituent cells. The present invention fills this need and other similar applications as well.
The developmental biology of the pancreas organ and especially the endocrine cells comprising beta islets has been extensively studied, and this research indicates clearly that insulin-producing beta cells normally arise from endodermally-derived precursor cells through a complex series of regulatory events involving coordinated expression of a variety of helix-loop-helix transcription factors regulating endoderm establishment and pancreatic determination. The morphogenesis of the pancreas gland as well as epithelial-mesenchymal interactions that are instrumental in pancreas development have also been described (Reviewed by Edlund, 2001 & Jensen, 2004).